Funny-Named Mailbox Folders

I set up my email accounts on a new MacBook and a bunch of oddly named folders now appear under the various accounts. (in one case the mailbox name was a password I use.) and most of these mailboxes are empty btw and if they aren't, the mail is often mail addressed to a different account altogether.

what caused this and how can I fix it?

I'm using Mojave 10.14.1